1. Field of the Invention
The present invention relates to a liquid crystal display device, and especially relates to a liquid crystal display device that performs gradation display by a combination of a plurality of sub-frames according to a gradation level expressed by a plurality of bits.
2. Description of the Related Art
Conventionally, a system of driving sub-frames is known as a half-tone display system in a liquid crystal display device. In the system of driving sub-frames that is one type of a time axis modulation system, a predetermined period (for example, one frame as a display unit of one image in a case of a moving image) is divided into a plurality of sub-frames, and driving of each pixel is performed by a combination of the sub-frames in accordance with a gradation to be displayed. The gradation to be displayed is determined based on a ratio of a driving period of a pixel to a predetermined period. The ratio of a driving period of a pixel to a predetermined period is determined by a combination of the divided sub-frames.
As a liquid crystal display device that employs the system of driving sub-frames described above, one disclosed in Japanese Translation of PCT International Application Publication No. JP-T-2001-523847 is known, for example, in which each pixel is configured from a master latch, a slave latch, a liquid crystal display element, and first to third as a total of three switching transistors. In this case, in each pixel, one-bit first data is applied to one of two input terminals of the master latch through the first switching transistor; and one-bit second data that is in a complementary relation with the first data is applied to the other input terminal through the second switching transistor. Then, when an object pixel is selected based on an application of a row selection signal through a row scanning line, these first and second switching transistors are turned ON, and the first data is written. When the first data is a logical value “1,” and the second data is a logical value “0,” display based on the pixel data is performed.
After data are written to all of the pixels by an operation as described above within a sub-frame period, third switching transistors of all of the pixels are turned ON in the sub-frame period. Then, the data written in the master latch are simultaneously read out to the slave latches. Then, the slave-latched data applies the data latched in the slave latch to the pixel electrode of the liquid crystal display element. The series of operations are repeated for each sub-frame, and desired gradation is performed based on a combination of all of the sub-frames in one frame period.
That is, in the liquid crystal display device that employs the system of driving sub-frames, with respect to all of the sub-frames existing in one frame period, the same or a different predetermined display period is allocated to each sub-frame. Then, each pixel performs white display in all of the sub-frames at the maximum gradation display (display is performed); while each pixel does not perform white display in all of the sub-frames at the minimum gradation display (non-display, that is, black display is performed). In cases other than the maximum gradation display and the minimum gradation display, sub-frames to be white-displayed are selected according to a gradation to be white-displayed. Note that the conventional liquid crystal display device uses a digital driving system having a two-stage latch configuration, where input data is digital data that indicates a gradation.
Here, typically, in a liquid crystal display device, it is necessary to perform AC drive in which positive and negative voltages are alternately applied to a common electrode of a substrate, which opposes a pixel electrode, by about 1 kHz in order to prevent deterioration of the liquid crystal element (for example, deterioration due to burning). At this time, it is necessary to invert the polarity of the pixel electrode in accordance with the polarity of the common electrode. However, in the liquid crystal display device using the above digital driving system having a two-stage lath configuration, inverted data needs to be rewritten again with respect to a pixel circuit.
Therefore, for example, Japanese Translation of PCT International Application Publication No. JP-T-2002-515606 W discloses a liquid crystal display device that has a configuration in which a pair of voltage supply terminals that supply a voltage corresponding to the positive and negative polarities are provided in addition to a pixel circuit, and in which a multiplexer for selectively connecting one or the other of the voltage supply terminals is included. According to the configuration, it is not necessary to rewrite the inverted data again, and the polarity of the pixel electrode can be inverted in accordance with the polarity of the common electrode.
However, in the conventional liquid crystal display device as described above, each of the two latches in each pixel is configured from a static random access memory (SRAM) and therefore, the number of transistors that configures the circuit increases. In addition, if the configuration includes a multiplexer in addition to the two latches, the number of transistors further increases. Therefore, there is a problem that the downsizing of the pixel is difficult.
Meanwhile, in a case where a configuration not including the multiplexer is employed in order to decrease the number of transistors, the AC drive is performed by rewriting the inverted data into the pixel circuit. In this case, there is a difference between application times of the positive polarity and the negative polarity depending on the gradation of the pixel, and thus display deterioration due to burning may occur. To avoid the problem, there is a method of equalizing the positive and negative application times by driving, using a pair of equal positive and negative sub-frame periods. However, there are problems that the number of combinations of sub-frames is decreased by half, and the gradation performance is deteriorated.
There is a need to at least partially solve the problems in the conventional technology.